Mattress spring core construction over the years has been a continuously improving art with advancements in materials and machine technology. A well known form of spring core construction is known as a Marshall spring construction wherein metal coil springs are encapsulated in individual pockets of fabric and formed as elongate or continuous strings of pocketed coil springs. In an earlier form, these strings of coil springs were manufactured by folding an elongate piece of fabric in half lengthwise to form two plies of fabric and stitching transverse and longitudinal seams to join the plies of fabric to define pockets within which the springs were enveloped.
More recently, improvements in spring core constructions have involved the use of fabrics which are thermally or ultrasonically weldable to themselves. By using such welding techniques, these fabrics have been advantageously used to create strings of individually pocketed coil springs wherein transverse and longitudinal welds, instead of stitching, are used to form the pockets encapsulating the springs.
Once strings of pocketed springs are constructed, they may be assembled to form a spring core construction for a mattress, cushion or the like by a variety of methods. For example, multiple or continuous strings may be arranged in a row pattern corresponding to the desired size and shape of a mattress or the like, and adjacent rows of strings may be interconnected by a variety of methods. The result is a unitary assembly of pocketed coil springs serving as a complete spring core assembly.
Conventional pocketed spring cores incorporating pocketed strings of springs typically use springs of uniform diameter and pitch. U.S. Pat. No. 5,868,383 discloses a pocketed spring assembly incorporating springs which have different diameters and different pitches. An end portion of the pocketed spring may substantially compress before a middle portion compresses. The end portion may compress without the middle portion compressing when subject to lighter loads. However, due to the seams separating individual pockets extending the full height of the string of springs, the advantages of the coil springs may not be fully realized. The capability of the individually pocketed springs to act independently from the adjacent pocketed springs in the string of springs may be compromised.
Therefore, there remains a need to combine multiple technologies to improve the feel and motion transfer in a bedding or seating product having a pocketed spring core.